Final answer:
Matter can exist as a solid with a definite shape and volume, a liquid with a definite volume but adaptable shape, or a gas with neither definite shape nor volume. Changes of state, such as melting or evaporation, are physical changes that don't alter the chemical makeup of a substance. Density is a key intensive property that describes mass per unit volume.
Step-by-step explanation:
The state a substance takes, whether as a solid, liquid, or gas, is an essential physical property. A solid has a definite shape and volume, characterized by closely arranged particles that are fixed into place. Liquids also have a definite volume but adapt to the shape of their container, with particles that move more freely than in solids. Gases, conversely, have neither a definite shape nor volume, expanding to fill their container, with particles that are far apart and in constant chaotic motion. When a substance changes from one state to another, such as solid to liquid or liquid to gas, it is undergoing a physical change. These changes are usually the result of a temperature change, but they don't alter the substance's chemical composition.
A homogeneous mixture is uniform throughout and includes solutions where different substances are dissolved and evenly distributed. On the other hand, a heterogeneous mixture consists of regions with different properties and compositions. Chemical changes involve a substance reacting to form a new one, such as when iron rusts.
Density is an important intensive property of matter, indicating the mass per unit volume of a substance, which varies from solids to liquids to gases. Understanding these properties and changes helps to predict and explain the behavior of substances in different states.